1. Field of the Invention
The invention relates generally to subsea data communications systems and methods. More particularly, the invention relates to a subsea data telemetry systems and methods that leverage sea floor rig structures to enhance the quality of the telemetry data.
2. Background of the Technology
Modern petroleum drilling and production operations demand a great quantity of data and information relating to downhole conditions and drilling and production parameters. Such information typically includes characteristics of the earth formations traversed by the wellbore, as well as data relating to the size and configuration of the borehole itself. The collection of information relating to downhole conditions, commonly referred to as “logging”, can be performed by a variety of methods.
In conventional oil well wireline logging, a probe or “sonde” that houses formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The upper end of the sonde is attached to a conductive wireline that suspends the sonde in the borehole. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. The instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
One problem with obtaining downhole measurements via wireline is that the drilling assembly must be removed or “tripped” from the drilled borehole before the wireline can be lowered downhole to obtain the desired borehole information. This can be both time-consuming and extremely costly, especially in situations where a substantial portion of the well has been drilled. For example, thousands of feet of tubing may need to be removed and stacked on the platform. Typically, drilling rigs are rented by the day at a substantial cost. Consequently, the cost of drilling a well is directly proportional to the time required to complete the drilling process. Removing thousands of feet of tubing to insert a wireline logging tool is often an expensive proposition.
As a result, there has been an increased emphasis in recent years on the collection of data during the drilling process. Collecting and processing data during the drilling process eliminates the necessity of removing or tripping the drilling assembly to insert a wireline logging tool. It consequently allows the driller to make accurate modifications or corrections as needed to optimize performance while minimizing down time. Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well are generally referred to as “measurement-while-drilling” techniques, or “MWD.” Similar techniques, concentrating more on the measurement of formation parameters, are generally referred to as “logging while drilling” techniques, or “LWD.” While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term MWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
When oil wells or other boreholes are being drilled, it is frequently necessary or desirable to determine the direction and inclination of the drill bit and downhole motor so that the assembly can be steered in the desired direction. Additionally, information may be required concerning the nature of the strata being drilled, such as the formation's resistivity, porosity, density, and its measure of gamma radiation. It is also frequently desirable to know other downhole parameters, such as the temperature and the pressure at the base of the borehole. Once this data is gathered at the bottom of the borehole, it is necessary to communicate it to the surface for use and analysis by the driller.
Most conventional offshore MWD systems include sensors or transducers located at the lower end of the drill string and a receiver or detector located at the sea surface. Typically, the downhole sensors employed in MWD applications are positioned in a cylindrical drill collar that is positioned close to the drill bit. While drilling is in progress, the sensors continuously or intermittently monitor drilling parameters and formation data and transmit the information to the surface detector by some form of telemetry. Thus, in the telemetry system, data acquired by the sensors is transmitted from the borehole bottom to the receiver located on the surface.
There are a number of known telemetry systems that transmit information regarding downhole parameters to the surface without requiring the use of a wireline tool. Of these, mud pulse systems are one of the most widely used telemetry systems for MWD applications. Other forms of telemetry include electromagnetic (EM) telemetry and acoustic telemetry. However, offshore MWD operations pose numerous difficulties to conventional mud pulse, acoustic, and EM telemetry systems and methods and often impose undesirable limitations on data transfer rates and/or depth ranges.
Accordingly, there remains a need in the art for improved systems and methods for communicating MWD and LWD data in offshore applications. Such systems and methods would be particularly well received if they offered the potential to increase data transmission rates between the borehole bottom and the surface and/or enhance data quality.